This invention relates to an improved map reading plotter and, in particular, to a plotter for providing a direct conversion between grid and magnetic azimuth readings without having to perform arithmetic calculations.
As disclosed by Barry in U.S. Pat. No. 3,126,151 and Gray in U.S. Pat. No. 2,967,016, devices for converting true heading readings to magnetic value or the reverse thereof have been known for quite some time in the art. These devices all take a similar form in that concentric discs of different diameters are rotatably supported upon a common pivot pin. Each disc contains its own compass rose printed thereon that allows the user to set values obtained from some independent source, such as a compass or a chart, into the device. The outer or larger disc generally remains fixed and provides true directional values as typically read from a chart or map. The smaller inner discs provide correction values that are set into the device in various ways to permit comparative readings to be made between the circular scales. As can be seen, these prior art devices operate in the same sense as a circular slide rule whereby the accuracy attained is dependent to a large extent upon the user's ability to make the proper entries and to carry out the proper conversion steps.
While these prior art devices, to some extent, preclude the navigator from having to carry out some arithmetic calculations, and oftentimes reduce the number of factors that might lead to a human error, it should be noted that they are generally only useful as an intermediate aid in processing the conversion between values obtained from a compass or a chart. Gray, in the above-noted patent, attempts to use his device as a plotter by providing it with an extended straight edge. However, because all the elements of the device are constructed of metal or an opaque material, the device itself obstructs the user's view of the chart and thus severely affects the degree of accuracy that can be attained when used as a direct reading instrument. It should also be noted that the disc is secured to a common pivot pin which further limits its value as a direct reading plotter.